1. Field of the Invention
The present invention is directed to a process and apparatus for positioning an object by means of a drive having at least two fixed driving speeds and comprising an electric motor which, prior to being switched to operate at the next lower driving speed from the approximately highest driving speed, is switched off at a predetermined switch-off location in advance of a given destination for the object and is braked until reaching the next lower driving speed.
2. Description of Related Art
The Lenord & Bauer GEL Model 7100 Positioning Controller is a known positioning apparatus with a pole changing three-phase motor having several fixed driving speeds. During positioning, in order to switch to the next lower driving speed from the highest driving speed, the drive is switched off at a predetermined distance before a given destination and then braked until the next lower driving speed is attained or reached. When using this positioning controller, the distance between the switch-off locations and the respective destination is always identical for all permissible drive loads for one and the same driving speed. Different drive loads, however, effect or change the acceleration behavior and braking behavior of the positioning apparatus, and this positioning process must accordingly be intercepted or interrupted within the deceleration path by the inclusion of buffer paths, the lengths of which correspond with the individual fixed driving speeds. The buffer path lengths are selected so that the next lower speed may still be switched to at the maximum allowed drive load such that the destination may eventually be approached by proceeding at the slowest available driving speed to achieve a high target accuracy. As a result of the very long buffer paths thus, required, this prior art positioning process is relatively slow.
Conventional apparatus and processes for positioning an object device by means of a frequency converter for a three-phase motor controlled by a distance sensor are also known. A given destination is set by the frequency converter which changes the rate of rotation of the motor based on the remaining distance from the destination such that the device arrives at the target position in a highly accurate manner. This prior art apparatus and process is disadvantageous, however, in that the frequency converter is relatively expensive and sensitive.
Thus, the principal object of the present invention is to provide a process and apparatus for relatively rapid positioning of an object with high positioning accuracy over a relatively extended load range by means of a drive having at least two fixed driving speeds and utilize a relatively inexpensive pole changing three-phase asynchronous motor or induction motor. In the inventive positioning technique the motor switches off and is braked from the highest respective driving speed at a calibrated, predetermined switch-off location in advance of a given destination and is subsequently switched to the next lower drive speed without the need for unwanted buffer paths (which would otherwise increase the positioning time) to achieve relatively high positioning accuracy.